The olfactory bulb projects directly to the neocortical part of insula cortex in opossum, while the olfactory projections to a similar zone in rat are mediated by piriform cortex. The thalamic relationships of the olfactory zone of neocortex in opossum were sought by injecting horseradish peroxidase (HRP) (a retrograde tract tracing method) into this neopallial zone. Neuron cell bodies filled with HRP reaction product were found clustered tightly in thalamus in an area, which in rat, corresponds to the medial part of the ventromedial nucleus.